1. Field of the Invention
The present invention relates to a system for exposure measurement for the optical instrument such as camera and/or focus detection for the image forming optics.
2. Description of the Prior Art
Among the conventional exposure measurement system for optical instrument such as camera in which the light measuring method suited for the photographic field at the time of taking photograph can be chosen there is one which is so constructed that a plural number of light sensing elements such as CdS cells intended for the spot light measurement and another plural number of light sensing elements such as CdS cells intended for the mean light measurement are provided at the positions different from each other whereby at taking photograph either the spot light measurement or the mean light measurement is selected in accordance with the conditions of the photographic field at taking photograph by comparing the output of the one light measuring method with that of the other light measuring method so as to carry out the suitable exposure measurement.
One of the example is the camera disclosed in U.S. Pat. No. 3,690,241, in which camera the first light sensing element is disposed so as to be brought in front of the film while the second light sensing element is disposed so as to be opposed to the plane of the pentagonal prism composing a view finder optics of the camera through which plane the light beam goes out of the pentagonal prism whereby the first light sensing element serves as that for the so called spot light measurement responsing to a comparatively small part of the light beam to be exposed to the film while the second light sensing element serves as that for the so called mean light measurement responsing to almost all the light beam coming from the photographic field. Hereby the first and the second light sensing element can be connected with the same exposure measurement circuit selectively by means of the manual operation from outside of the camera. Therefore it is possible for a photographer to select either the spot light measurement or the mean light measurement in accordance with the then condition of the photographic field so as to carry out a suitable exposure measurement by selectively connecting the first or the second light sensing element to the exposure measuring circuit and comparing the output of the one light measuring method with that of the other light measuring method for example by means of meter.
However, in case of the exposure measuring system constructed as mentioned above, the light sensing element for the spot light measurement is disposed at a certain determined position with regard to the image plane so that it is disadvantageous that the light beam only at a certain determined point of the object in the image plane could be measured, whereby for example in case of the single reflex camera there is a restriction that the range capable of the spot light measurement is determined in advance because the spot light measurement can not be carried out at any optical position in the object, of the image plane in the view finder.
On the other hand, various methods respectively devices for detecting the focus point of the image forming optics by utilizing the photoelectric characteristics of the photoelectric converting elements have so far been proposed, whereby what is theoretically considered to be able to obtain the focus point with comparatively high accuracy is the method as proposed by the Japanese Patent Publication No. Sho 42-14096 according to which a plural number of the parts having a fine but proper area are provided on the focus plane so as to obtain the electrical output by converting the light amount of each fine part by means of the photoelectric converting element in such a manner that the differences of the electrical outputs between two adjacent fine parts are converted in absolute values or squared value and then added whereby the focus point is considered to be obtained when the added value is maximum.
However, in case of the device disclosed in the Japanese Patent Publication No. Sho 42-14096 the analog amount of the output of each photoelectric converting element is processed and therefore the composition of the signal processing circuit for processing the output of each photoelectric converting element is much complicated so that the exact detection is practically impossible due to the errors taking place at processing the output signals, which is disadvantageous. Hereby further the composition of the circuit is much more complicated according as the number of the photoelectric converting elements increases so that the number of the elements to be used is necessarily restricted.
Further, as is disclosed for example, in the Japanese Utility Model Publication No. Sho 48-43379, a device in which a light sensing means consisting of a plural number of fine light sensing elements is used for the focus point detection and the exposure measurement is so far known whereby in case of the device disclosed in the Japanese Utility Model No. Sho 48-43379, a method according to analog control is adopted according to which method the analog amount of the output of each light sensing element in the light sensing member is processed so as to control the focus point detecting and exposure measuring device. However, in the analog control there is a theoretical instability in such a manner that in case one light sensing means is used for the focus detection and for the exposure measurement the composition of the electrical circuit becomes complicated, whereby the error becomes larger at the time of the focus point detection and of the exposure measurement so that the exact measurement is practically impossible. Further, in case the light sensing system is composed of a plural number of the light sensing elements, the composition of the circuit becomes much more complicated according as the number of the light sensing elements increases so that the number of the light sensing elements is necessarily limited, which makes the exact measurement all the more difficult.